Catalysts For Oxidative Coupling Of Methane Research Articles

Influence of the La/Ce ratio on La-Ce oxides promoted by Sr in the methane oxidative coupling reaction

The high methane availability and its significant role as a greenhouse gas have led the scientific community to pursue methods for its use. This paper proposes the usage of the oxidative coupling of methane (OCM) as a promising path to transform methane directly into value-added hydrocarbons, mostly ethylene, which serves as a crucial compound in the petrochemical sector. An efficient OCM catalyst should contain substantial basicity and oxygen vacancies for providing surface electrophilic oxygen species, such as superoxides and peroxides, instrumental for boosting selectivity towards C2 products. Mixed lanthanum-cerium oxide (La-Ce) catalysts have emerged as strong candidates in OCM due to their high thermal stability, oxygen mobility, and high alkalinity. In this study, they were prepared through a surfactant-assisted hydrothermal method with different La/Ce ratios for fine-tuning their basic properties and promoting oxygen mobility on the surface of the catalysts. Samples followed a volcano-shaped relationship between C2 yield and La content, with optimal performance at a La/Ce molar ratio of 2.1, attributed to the interplay between the high amount of basic sites and oxygen vacancies, increasing the presence of superoxide species over lattice oxygen. Moreover, the Sr-promoted catalyst showed high density of strong basic sites while preserving the reactive oxygen species, achieving 20 % CH4 conversion, with 57 % C2 compounds selectivity at 750 °C and GHSV of 18.000 mL.gcat−1.h−1.

The combined effect of the oxidizing agent and its concentration on the oxidative coupling of methane

The combined use of O2 and CO2 as oxidizing agents in the oxidative coupling of methane (OCM) has been assessed over two representative OCM catalysts, i.e., La‐Sr/CaO and NaMnW/SiO2, under a wide range of inlet O2 and CO2 concentrations. The overall impact of CO2 was found to depend on the operating conditions and the catalyst used. At O2‐rich conditions, a negative effect of CO2 was observed on C2 selectivity. At O2‐lean conditions, a positive effect of CO2 on both catalysts was observed, likely originating from an enhanced dehydrogenation of C2H6 to C2H4 and limited deep oxidation of the involving hydrocarbons and reaction intermediates. The behavior induced by CO2 is attributed to its mild oxidizing ability and interactions with the catalysts. The enhanced dehydrogenation of C2H6 was observed and confirmed by specific tests with a CO2 and C2H6 feed. Well‐controlled CO2 addition improved the value of the OCM product mixture, mainly due to a 9% increase in C2H4 and a 19% increase in CO selectivity, resulting in a 25% reduction of CO2 at the outlet over the La‐Sr/CaO catalyst. These findings provide new insight into the CO2 effect on OCM and the data to translate this research into OCM process alternatives.

Model-based catalyst screening and optimal experimental design for the oxidative coupling of methane

The oxidative coupling of methane (OCM) to produce ethane and ethylene (C2 compounds) as platform chemicals involves complex chemistry with reactions both in the gas phase and on the catalyst surface, resulting in a distribution of products at the expense of C2 selectivity. This work uses experimental data from a variety of mixed metal oxides on supports at different reaction conditions (temperature, contact time, and reactant flow rates) to train a random forest regressor that predicts methane conversion and C2 selectivity (key performance indicators (KPIs)). The kinetically validated random forest models are deployed to locate optimal conditions that maximize C2 yield for each of the catalysts. Investigating the regressor interpretability via feature importance reveals that the choice of metals and support are crucial to C2 selectivity predictions in addition to the reaction conditions, while the predictions of methane conversion are largely governed by the reaction conditions. The machine learning (ML) regressor is used as a kinetic surrogate to find a locus of optimal reaction conditions that maximize both selectivity-conversion for each of the catalysts via a multi-objective optimization routine. The maximum C2 yields for catalysts are projected to be improved by 15% on average. Analyzing the catalysts with respect to a popular OCM catalyst, Mn-Na2WO4/SiO2, using the optimal locus eliminates variability in the process conditions to reveal distinct patterns based on intrinsic properties of metals and supports. Further, the decision space with catalyst descriptors and reaction conditions is optimized for high C2 yields using the ML surrogate, in a static multi-objective optimization routine, and an adaptive Bayesian routine, where the latter was found to have a wider field focus in proposing catalyst formulations and reaction conditions. Transition metal oxides on a variety of supports were proposed but not their lanthanide oxide counterparts. The framework has the potential to lend itself to materials acceleration platforms where it is crucial to consider multi-scale phenomena that impact downstream KPIs.

Alkaline Earth Metal Aluminate Support for Selective Oxidative Coupling of Methane

AbstractThe oxidative coupling of methane (OCM) provides a direct route to transform methane into higher value products. The alkali metal tungstate catalysts have demonstrated high selectivity towards C2 products, ethane (C2H6) and ethylene (C2H4). However, the severe sintering of the SiO2 support limits the reaction rate requiring active site densification to compete with unselective gas phase combustion reaction especially at high pressure operations. This work studies alkaline earth metal oxides as supports for K2WO4 catalysts to maintain comparatively high surface area under OCM conditions. Among Mg, Ca, and Sr aluminates, the K2WO4/MgAl2O4 catalyst exhibited the highest C2 yields. To achieve high C2 product selectivity, a relatively large loading of K2WO4 (20 wt %) was required likely to cover the unselective surface sites of MgAl2O4 support. The catalyst further showed high levels of stability when utilized at high pressures (0.9 MPa) for over 60 h, without any change in product selectivity. The Mn/K2WO4/MgAl2O4 showed multifold higher CH4 conversion rate, compared with the SiO2 counterpart. The findings showcase the potential of the MgAl2O4 support as a viable candidate for alkali metal tungstate catalysts for OCM, which introduces high density of active components in given volume in the reactor, which induces high contribution of the catalyst relative to the pure gas phase oxidation.

Boosting the generation of key intermediate methyl radical (CH3•) in OCM reaction on magnesium oxide catalysts by regulating the electronic state of the active site

Methyl radical (CH3•) is the critical precursor to generate C2 hydrocarbons in oxidative coupling of methane (OCM) reaction. It is crucial to explore how to efficiently generate CH3• and reveal the intrinsic principles behind it. In this study, we select MgO with a relatively simple structure as the research object, and probe the effects of the electronic state of active site on the CH3• generation by the density functional theory (DFT) calculation. The results show that the pure MgO with the different crystal surfaces as the benchmark models exhibit poor catalytic performance for CH3• generation; while Li doped MgO surface or subsurface that have the electron-deficient of active sites, exhibit excellent catalytic performance for it, which is attributed to that the electron-deficient of active sites are easier to receive electron of H and thereby promote the efficient generation of CH3•; pure MgO surface or subsurface with oxygen vacancy as well as Li doped MgO surface or subsurface with oxygen vacancy as the contrast models that have the electron-rich of active sites, exhibit poor catalytic performance. It can be obtained that the active site with electronic-deficient state is the key to the efficient generation of CH3•. The study can provide some guidance for designing high efficient OCM catalysts.

Indirect design of OCM catalysts through machine learning of catalyst surface oxygen species

Catalysts for oxidative coupling of methane (OCM) were designed through machine learning of a property of surface oxygen species on the basis of the knowledge that catalytic performance for the OCM is affected by catalyst surface oxygen species.

Na2WO4/Mn supported on all-silica delaminated zeolite for the optimal oxidative coupling of methane via the effective stabilization of tetrahedral WO4: Elucidating effects of support precursors with different crystal structures, Al-addition, and morphologies

The oxidative coupling of methane (OCM) converts this abundant natural feedstock into value-added products and is typically performed in the presence of catalysts such as SiO2-supported Na2WO4/Mn to suppress the undesired deep oxidation affording CO and CO2. Currently, the phase transformation of SiO2 supports to α-cristobalite is known to be important for securing high OCM activity, although the underlying reasons of this influence remain debatable. Herein, Na2WO4/Mn catalysts supported on several SiO2-based materials including conventional amorphous SiO2 and crystalline zeolites were prepared to close the above knowledge gap and thus promote the design of more efficient OCM catalysts. The best support was identified as calcined D-ITQ-1, which is a well-developed delaminated zeolite with a thin basal all-silica nanosheet structure facilitating the transition to the α-cristobalite phase. The corresponding catalyst retained high activity over 100 h and exhibited one of the highest yields (25.8%) of C2+ hydrocarbons (paraffins and olefins) reported for dopant-free Na2WO4/Mn/SiO2 catalysts to date. The results of XRD, Raman, and 29Si NMR indicated that this high performance can be ascribed to the stabilization of tetrahedral WO4 units due to the transition from uncalcined silica to α-cristobalite. Thus, the present work advances our understanding of structural phase transitions and the concomitant formation of the desired active species in Na2WO4/Mn/SiO2 catalysts and paves the way to the design of better catalysts through the modification of zeolite supports.

Catalytic Routes for Direct Methane Conversion to Hydrocarbons and Hydrogen: Current State and Opportunities

Natural gas, the cleanest fossil fuel, is an abundant source of methane and expected to play an increasingly important role in powering the world’s economic growth over the energy transition of the coming decades. Methane has the potential to be a CO2-free feedstock to cogenerate hydrogen (H2) and added value “building-blocks” chemicals (e.g., olefins and aromatics) for petrochemistry. In this review, the two processes (i) the oxidative coupling of methane (OCM) for production of ethylene and (ii) the nonoxidative methane dehydroaromatization (MDA) producing hydrogen and benzene are discussed. Both routes convert methane directly into valuable products, an advantage over the several-steps syngas route. The performances of various a variety of catalysts reported during the last 25 years for OCM (MnNaW, La2O3, Li-MgO, etc.) and MDA (M/HZSM-5, M/TNU-9, M/IM-5, M/ITQ-2, M@SiO2, M@CeO2, TaH/SiO2, GaN/SBA15, single-site M@HZSM-5, bimetallic M-M′/HZSM-5, core–shell structures, M/Zr(SO4)2 with M = Mo, Fe, Pt) under similar reaction conditions are compared. The major drawbacks and the strategies used to mitigate the main challenges related with the performance of the catalysts in both OCM and MDA reactions are critically revealed. For instance, the overoxidation in the OCM is mitigated by optimizing of the operating conditions, using alternative oxidants, and the application of membrane reactor technology are discussed. In the MDA reaction, the major issue is the catalyst deactivation by coke formation and migration and sintering of metallic active phases. Strategies for robust catalysts, methods for mild coke removal, pretreatment under reductive atmosphere are presented. Approaches to improve aromatics yields over coke production by addition of promoters or co-feed reactants to the MDA catalysts are also discussed.

Atomically dispersed SrOx species on exposed {2 2 2} facets of pyrochlore La2Zr2O7 nanocrystals for boosting low-temperature oxidative coupling of methane

The efficient catalysts of strontium oxide (SrOx) supported on exposed {222} facets of pyrochlore La2Zr2O7 nanocrystals (SrOx/LZO-Y) were elaborately synthesized by incipient wetness impregnation method. The introduction of atomically dispersed SrOx species on La2Zr2O7 surface can increase the amount of surface oxygen vacancies induced by Sr2+ substitution of La3+ cations, and the increasing of moderate surface alkaline sites are beneficial to enhance catalytic performance for oxidative coupling of methane (OCM). The SrOx/LZO-Y catalysts have super catalytic performance during low-temperature OCM reaction. SrOx/LZO-12 catalyst with the highest concentrations of O2– species exhibits the highest activity and the lowest apparent activation energy (84 kJ mol−1) during low-temperature OCM into C2H4 and C2H6 (C2) products, i.e., its CH4 conversion, selectivity and yield of C2 products at 600 °C are 30.5 %, 47.4 % and 14.5 %, respectively, and its ratio of C2H4/C2H6 products is 0.77. Based on the results of multiple experimental characterizations, the mechanism of SrOx/LZO-Y catalysts for OCM reaction is proposed: the surface alkaline active centers (O2–) induced by doping of Sr ions can promote the critical step of CH4 activation significantly during low-temperature OCM reaction. It is a new strategy to develop the low-temperature and high-activity catalysts for OCM in practical application.

Ethylene Production from Oxidative Coupling of Methane in Solid Oxide Electrochemical Cells

The oxidative coupling of methane (OCM) to ethylene is a potentially more economical and environmentally benign approach for ethylene production compared to conventional high temperature thermal cracking of natural gas. The OCM reaction is typically conducted in packed bed flow reactors under thermal conditions in the presence of a heterogeneous catalyst. However, due to the intrinsic thermodynamic and kinetic constraints, the existing catalytic systems operated in packed bed reactors have not yet met the techno-economic requirements for the commercialization of this process. As an alternative, membrane reactors that selectively conduct the oxygen ions (O2-) could potentially offer significantly higher methane conversion and C2 product selectivity compared to conventional packed bed reactors. The lower partial pressure of oxygen and the distinct nature of oxygen species available for methane oxidation in the membrane reactor could effectively suppress the kinetics of the chemical transformations leading to the combustion products. In this context, solid oxide electrochemical cells (SOEC), specifically based on oxygen ion conducting membranes, are excellent platforms for conducting the OCM reaction. Here we demonstrate the development of novel SOECs integrated with a series of advanced OCM catalysts to achieve highly efficient ethylene production.

Elucidating the effect of barium halide promoters on La2O3/CaO catalyst for oxidative coupling of methane

The industrialization of oxidative coupling of methane (OCM) is restricted by the low once through yield of C2 hydrocarbons. Recently, the halogen-assisted OCM process has been attempted to overcome this issue, but the reaction stability was poor due to the rapid loss of gas-phase halides or molten alkali halides. In this work, the barium salts, particularly barium halides (BaCl2 and BaF2), were demonstrated to be efficient promoters to improve the OCM reactivity of La2O3/CaO catalyst by increasing both C2 selectivity and C2H4/C2H6 ratio, and simultaneously achieving outstanding reaction stability. The promoting mechanism can be understood in two aspects. On the one hand, the introduction of barium salts increased the amount of surface electrophilic oxygen species, serving as the alkaline active sites for selective methane activation. On the other hand, the barium halide additives induced the in-situ formation of methyl halide intermediates facilitating C2H6 dehydrogenation, and their intimate contact with catalyst substrate restricted the rapid halogen loss and thereby improved the catalytic stability. This work not only provides a class of efficient OCM catalyst, but also offers a highly stable halogen-assisted reaction strategy.

Machine Learning-Aided Catalyst Modification in Oxidative Coupling of Methane via Manganese Promoter

To enhance CH4 conversion values in oxidative coupling of methane (OCM) reactions under an O2-lean condition (CH4/O2 = 6.0), a support vector regression (SVR) and one-hot encoding manner implemented machine learning (ML) is examined. From an open-source high-throughput screening (HTS) database of 300 OCM catalysts made by random sampling from a materials space, the top 10 three-element-supported catalysts with C2 yield higher than 11.0% and C2 selectivity higher than 80.0% at CH4/O2 = 6.0 were selected as targets for modification. Then, ML-aided investigation of an additive fourth element as a promoter was performed at the SVR field based on the HTS database among 350 catalysts (40,330 data points). Application of one-hot encoding to ascertain positive elements for CH4 conversion revealed that manganese (Mn) frequently appears at CH4 conversion higher than 44.0%. After the 10 selected catalysts were prepared with the Mn additive, their OCM performance was compared with those of pristine three-element-supported catalysts. Results show that four catalysts represent positive features on C2 yield in the presence of additive Mn working as a promoter. Consequently, 5 wt % Mn-loaded LiFeBa/La2O3 and LiBaLa/La2O3, respectively, show attractive OCM performance of 16.3% C2 yield with 88.4% selectivity and 13.8% C2 yield with 71.9% selectivity, even under an O2-lean condition (CH4/O2 = 6.0).

A chemistry‐inspired neural network kinetic model for oxidative coupling of methane from high‐throughput data

AbstractA neural network kinetic model is developed for oxidative coupling of methane (OCM). The model is designed in cognizance of the underlying chemistry and associated reactor balance equations and trained on publicly available high throughput experimental data spanning a large material space of supported mixed metal oxide catalysts. The resultant model is then used to evaluate one of the most popular catalysts for OCM, viz. MnNa2WO4/SiO2, to understand the reaction kinetics and sensitivity of the catalyst to changing different components of the catalyst. The predicted activation barrier for methane conversion is 251 kJ mol−1, and the rate . Furthermore, the reference catalyst is local optimal as small changes to its composition, for example, by changing the individual metals or the support, did not improve (or often substantially reduced) methane consumption or the C2 formation rate.

Effects of Mg, Ca, Sr, and Ba Dopants on the Performance of La2O3 Catalysts for the Oxidative Coupling of Methane.

Oxidative coupling of methane (OCM) is a reaction to directly convert methane into high value-added hydrocarbons (C2+) such as ethylene and ethane using molecular oxygen and a catalyst. This work investigated lanthanum oxide catalysts for OCM, which were promoted with alkaline-earth metal oxides (Mg, Ca, Sr, and Ba) and prepared by the solution-mixing method. The synthesized catalysts were characterized using X-ray powder diffraction, CO2-programmed desorption, and X-ray photoelectron spectroscopy. The comparative performance of each promoter showed that promising lanthanum-loaded alkaline-earth metal oxide catalysts were La-Sr and La-Ba. In contrast, the combination of La with Ca or Mg did not lead to a clear improvement of C2+ yield. The most promising LaSr50 catalyst exhibited the highest C2+ yield of 17.2%, with a 56.0% C2+ selectivity and a 30.9% CH4 conversion. Catalyst characterization indicated that their activity was strongly associated with moderate basic sites and surface-adsorbed oxygen species of O2–. Moreover, the catalyst was stable over 25 h at a reactor temperature of 700 °C.

Catalyst-modified perovskite hollow fiber membrane for oxidative coupling of methane

The catalytic oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H6 and C2H4) represents one of the most effective ways to convert natural gas to more useful products, which can be performed effectively using La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) perovskite hollow fiber membrane microreactor. In this work, the effects of adding a thin BaCe0.8Gd0.2O3−δ (BCG) catalyst film onto the inner LSCF fiber surface as the OCM catalyst and a porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite layer onto the outer LSCF surface to improve the oxygen permeation were evaluated. Between 700 °C and 1000 °C, methane conversion increased in the order of uncoated, BCG and BSCF-coated, and BCG-coated LSCF hollow fiber while C2-selectivity and C2-yield increased in the order of BCG and BSCF-coated, uncoated, and BCG-coated LSCF hollow fiber. Oxygen permeation flux at the same temperature range, on the other hand, was enhanced in the order of uncoated, BCG-coated, and BCG and BSCF-coated LSCF hollow fiber. This finding demonstrates the complex interplay between oxygen permeation and OCM performance. The BCG and BSCF-coated hollow fiber was also subjected to thermal cycles between 850 °C and 900 °C for up to 175 hours during which the fiber showed minor degradation in oxygen permeation fluxes and relatively stable OCM performance.

Studies on activation factors for oxidative coupling of methane over lithium-based silicate/germanate catalysts

Activation factors for improving oxidative coupling of methane (OCM) catalytic performance have been identified. Potential OCM catalysts, Li4SiO4 and Li4GeO4, have been discovered.

Research Trends in Development of Highly Active Single Metal Oxide Catalyst for Oxidative Coupling of Methane

The conversion of methane to a value-added chemical is important for methane utilization and industrial demand for primary chemicals. Oxidative coupling of methane (OCM) to C2 hydrocarbons is one of the most attractive ways to use natural gas. However, it is difficult to obtain higher C2 yield in classic OCM reaction due to a favorable COx formation. Regarding this, various catalysts for OCM have been studied to fulfill desirable C2 yields. In this review, we briefly overview the single metal oxide types of OCM catalysts (alkaline-earth metal oxides and rare-earth metal oxides) and highlight the characteristics of catalysts in OCM reaction such as methane activation, surface basicity and lattice oxygen.

Oxidative Coupling of Methane for Ethylene Production: Reviewing Kinetic Modelling Approaches, Thermodynamics and Catalysts

Ethylene production via oxidative coupling of methane (OCM) represents an interesting route for natural gas upscaling, being the focus of intensive research worldwide. Here, OCM developments are analysed in terms of kinetic mechanisms and respective applications in chemical reactor models, discussing current challenges and directions for further developments. Furthermore, some thermodynamic aspects of the OCM reactions are also revised, providing achievable olefins yields in a wide range of operational reaction conditions. Finally, OCM catalysts are reviewed in terms of respective catalytic performances and thermal stability, providing an executive summary for future studies on OCM economic feasibility.

Waste-derived calcium oxide catalysts for oxidative coupling of methane

High-performance calcium oxide (CaO) catalysts were successfully synthesized from waste eggshell (ES) and waste cockle shell (CS), which consist mainly of calcium carbonate (CaCO3), and their catalytic behavior in the oxidative coupling of methane (OCM) was evaluated. Commercially available CaCO3 (CC) was also used as a starting material for comparison. The ES- and CS-derived CaO catalysts exhibited superior catalytic performance compared to the CC-derived catalyst due to presence of sodium (Na) and magnesium (Mg) elements in both ES and CS. Therefore, the concentration of Na and Mg in the waste-derived starting materials was further optimized, indicating that 5.0 wt.% of Na and 0.5 wt.% of Mg result in catalysts with high efficiency for OCM. Hence, this study clearly supports that waste materials can be converted into high-performance CaO catalysts for OCM by simply controlling their Na and Mg contents.

In-depth understanding of the crystal-facet effect of La2O2CO3 for low-temperature oxidative coupling of methane

Facet effect of lanthanum-based oxides on low-temperature oxidative coupling of methane (OCM) reaction have attracted great attention. In the present work, La2O2CO3 nanoparticles in the morphologies of triangular nanoplate and triangular nanocube with different ratio of {001} and {100} facets were prepared by facile hydrothermal method. The {001} facets exhibited higher OCM activity than {100} facets at 410 °C due to larger number of La3+–O2− sites, which could adsorb CO2 to form bidentate carbonate, and then reacting with oxygen to form adsorbed oxygen species. However, the higher methyl radical adsorption energy and the lower free energy of methane successive dehydrogenation of La2O2CO3{001} surface caused the excessive dissociation and deep oxidation of methyl radical, thus {001} facets show poor selectivity than {100} facets. Our findings will get a better understanding the facet effect of lanthanum-based oxides and provide guidance for the design of efficient low-temperature OCM catalysts.